The present invention relates to an apparatus and method for evaluating the electrical stress upon, and the response of, an implantable cardiac pacemaker to electromagnetic fields. More particularly, the present invention relates to an apparatus and hybrid method for evaluating the response of an implantable cardiac pacemaker which employs in vitro and bench testing.
Cardiac pacemakers (CPMs) have experienced significant technological advancements over the last decade, evolving from simple and bulky pulse generators to the small computerized units implanted in humans today. With the implementation of sensitive digital electronics in modern pacemaker designs, concerns have been expressed for the possibility of an increased sensitivity of CPMs to electromagnetic interference (EMI). To some extent these concerns have abated due to the increased sensitivity of CPM manufacturers to the EMI problem.
While the intent of the manufacturer may be to protect the CPM wearer from common and frequent sources of EMI, such as power line fields, microwave oven leakage, security system scanners, etc., there are other sources of EMI which should be accounted for in CPM designs. One of these sources of EMI is an electromagnetic pulse (EMP) simulator. Although it would be considered unlikely for the average CPM wearer to be subjected to a simulator's EMP, the effects of EMP on pacemakers is an important consideration for employees at EMP simulator sites, as well as CPM wearers in nearby public areas.
Previous EMI testing of pacemakers consisted of immersing pacemaker test samples in a saline-filled Plexiglas tank and subjecting them to a source of EMI. For EMP testing in particular, a timing circuit was used to fire the EMP within the appropriate pacemaker sense and refractory windows.
A CPM's sense window is a period of time in which the CPM is waiting for a natural heart beat. The refractory window is a period of time in which the CPM is simply waiting between heart beats and is not accepting external input. A CPM is constantly cycling through these windows. If a CPM detects a heart beat in the sense time window, the refractory period starts. If no heart beat is detected in the sense time window, the pacemaker generates a pulse to stimulate the heart, and then the refractory period begins. During a test, the continuous operation of the pacemaker is monitored, and diagnostics are performed before and after each EMI event.
The Association for the Advancement of Medical Instrumentation (AAMI) published in 1975 a "Pacemaker Standard" which specifically called for a rectangular test cell, or "phantom," that basically represents a typical male torso. The phantom was filled with a saline solution of a conductivity believed at the time to be consistent with the conductivity of human tissue at a defined frequency. The standard called for a pacemaker to be immersed in the center of the phantom with the CPM lead in a straight run parallel to the sides of the phantom and parallel to the incident E-field. A constant depth of 1 cm was to be maintained between the CPM lead and the front wall of the phantom.
Although the procedures set forth by the AAMI are consistent with the original objectives of the standard in most ways, they are not completely adequate for testing the effects of all EMI sources. The AAMI phantom does not consider any of the human extremities or surface contours and does not address the effects of the real earth interaction with the phantom and incident EMI.
The interaction of electromagnetic fields with the ground and the interaction of the human body with the ground (in the presence of electromagnetic fields) are important phenomenon for investigation when studying the effects of electromagnetic fields on implanted CPMs. Electromagnetic fields themselves can be attenuated or amplified when reflecting from a ground plane. Furthermore, the response of the human body to an electromagnetic field (.e. the amount of electromagnetic energy absorbed into the body) can be dramatically effected by contact or close proximity to the ground. The AAMI test cell was designed to be used in a free-field test scenario, and cannot accurately simulate a human standing on the ground for EM effects testing.
The "Pacemaker Standard" also calls for a saline solution with a conductivity of 0.267 S/m to be used as the test cell filler. However, the rationale for the use of this value of conductivity is unclear and appears to be an inappropriate choice for all EMI testing. Maintaining a uniform concentration of solute is another problem associated with filling a phantom with a saline solution.
In addition, the AAMI standard does not offer procedures or describe the apparatus for obtaining electrical stress data (currents and/or voltages on the pacemakers' lead(s)) for a pacemaker under test. Instead, the standard calls for monitoring electrodes to be placed in the test cell medium for the purpose of monitoring the pacemaker's output only, not the immediate electrical stresses input to the pacemaker. These electrodes can only qualitatively indicate the response of the pacemaker to the EM field (i.e. provide information as to whether the EM field had an effect on CPM operation). In order to quantitatively establish the effects of EM fields on pacemakers, electrical stress data must be obtained.